It would be nice to be able to set the start day of the month to when I get paid, that way I can see how much I’ve spent since pay day and it’ll be easier to track.
Could also be nice to support different pay frequencies (monthly, 4 weekly or weekly)
This will be a very useful feature! It’s been discussed a few times, here’s Hugo’s comments from the first topic -
https://community.monzo.com/t/set-start-of-month/1144/4?u=alexs
& a solution is on the way 
https://community.monzo.com/t/targets-custom-timeframes/6242/2?u=alexs